Proximity detectors determine the presence or absence of objects within the detection area of a presence sensor. A common use for a proximity detector includes detecting the presence of a person in a room to control a lighting fixture. Ideally, such a proximity detector determines when a person moves within a range of distances from the sensor, called an activation region, and activates the lighting fixture. Conversely, when the person moves out of the activation region, the lighting fixture is deactivated, often after a specific time delay.
Proximity detectors often measure the distance between an object and a sensor, for example, using time-of-flight calculations of a reflected pulse of light or sound, or angle-of-light detection. These are emitter-detector systems, where the detector emits a signal and a sensor detects characteristics of a received reflection of the signal. While such emitter-detector systems may determine the difference between a moving and a stationary object, they typically cannot distinguish an immobile person from an inanimate object, for example, a cardboard box. In this situation, if the detector is activated entirely by detecting the presence or absence of an object within an activation region, the light fixture of the previous example may remain illuminated if, for example, the cardboard box remains in the activation region after all persons have departed. Some proximity detector circuits may compensate for this deficiency by using timing circuits, for example, deactivating a lighting fixture when no motion is detected within the activation region over a period of time. However, this may result in deactivation of the lighting fixture when a person remains immobile within the activation region.
There are other examples where a traditional proximity detector may be inadequate. For example, a cat scampering across a room or a housefly flying past the sensor may trigger an unwanted change of state by the proximity detector.
It may be desirable for a proximity detector to have a configurable range. For example, a proximity detector used to activate and deactivate a computer monitor based on the detected proximity of a user may ignore a detected object located over five feet away from the monitor, but be activated by a detected object between one and five feet from the monitor. Another example is a smart-phone, where it may be desirable to de-activate and dim a touch sensitive screen when a user places the phone against his ear, but to re-activate the touch sensitive screen when the user moves the phone away from his ear. An additional example is a smart phone or other computational device, where hand or finger or body movements (gestures) are interpreted as commands to control the device.
The amount of energy used by an emitter-detector based proximity detector may also be problematic, as the emitter must expend energy by regularly generating signal pulses and receiving reflections to determine if a change has occurred within the activation region. For example, such power consumption concerns may be higher for battery powered devices. In addition, the signals generated by the emitter-detector may interfere with other electronic equipment operating nearby. Also, proximity detectors based on sensing visual spectrum or Near-infrared light may falsely trigger based on changes in ambient light, for example sunlight reflected through a window, or light from headlights of a moving car through a window.
Therefore, there is a need in the industry for a proximity detector that addresses the abovementioned shortcomings that is able to determine the presence of an object within an activation region, and discern if that object is human or not.